MET 452 Poster

MET 452 Poster

• 1.
  Investigating the CorrelationBetween Equatorial African Rainfall and Tropical Cyclone Activity in the North Atlantic Basin Michael Willette University of Northern Colorado, MET 452, Spring 2016 Introduction The North Atlantic’s tropical cyclone season has the potential to affect millions of people living along the eastern seaboard of the United States, Mexico, and the Caribbean. Predicting the variability of tropical cyclone activity each year is vital for human safety and the protection of economic interests. Although interdecadel oscillations, such as the NAO, may be responsible for some of this variability, the formation and movement of African waves that act as “seeds” for tropical cyclones is likely more important. This study acts to investigate this possible correlation between the North Atlantic tropical cyclone activity and rainfall anomalies in Equatorial Africa. Fig. 1. A figure from Jackson, Nicholson, and Klotter (2009) that shows the five-year mean number of lightning flashes for the year and for the SON season. These areas of lightning maximas are indicative of the genesis of MCCs and African waves that will move out to the Atlantic ocean, Figure 2: A storm track density analysis from Hopsch et al. ((2007) relating the origination of storm tracks in equatorial Africa to the positioning of tropical cylcone formation in the Atlantic ocean basin. Notice that the origination comes from two hotspots in northern and central Africa. Movement into the Atlantic Basin In order to get a comprehensive analysis of the link between tropical cyclone genesis in the North Atlantic basin and rainfall in equatorial (and specifically the Sahel region of Africa), the location of where these equatorial waves move off the western African coast into the North Atlantic is vital. Hopsch et. al. (2006) state that there is year-to-year variability in the positioning of the storm tacks of equatorial waves as they move off the coast of western Africa. Although SSTs and west African Rainfall can explain some of the variability, variation of the meridional wind shows the strongest positive correlation with tropical cyclone activity in year-to-year time scales. (Hopsch et. al., 2006) There ends up being two sources for storm tracks over the Atlantic; north and south of 15˚N in North Africa (Hopsch et. al., 2006), and that the southern storm track provides the highest amount of tropical cyclone activity. The Origination of African Waves The western portion of equatorial Africa undergoes some of the most unstable processes leading to intense thunderstorms and arguably some of the least understood. The climate is warm, wet, and volatile meteorologically. The area of maximum radiation from the sun and its associated ITCZ Studies show that wind patterns largely drive the location and intensity of these convective events. With the core at 600mb, the positioning of the African Easterly Jet corresponds well with convective and lightning maximums in equatorial Africa. (Jackson et. al., 2009) Specifically, the location of the right front quadrant of jet streaks associated with the African Easterly Jet appears to focus the most intense convective development. (Jackson et. al., 2009) Figure 3: A comparison of Burundi Highlands Precipitation Anolamlies from data produced by Bonnefille and Chalie (2000) on the left and a statistical model of tropical cyclone activity from Mann et al. (2009). Both sets of data were then compiled in Excel in this study. There are some correlations between the two plots, mostly being correlations when precipitation and tropical cyclone activity are both low. Discussion of Methods and Results To investigate this correlation, a comparison of data sets between two studies is used. Paleo-pollen proxy data from Bonnefille and Chalie (2000) is compared with a statistical model produced by Mann et al. (2009). The results are shown in figure 3 and some interesting correlations (or lack thereof) are seen. • Positive correlations are hard to find. A good correlation can be seen at 600 AD. • There seems to be some sort of lag in correlations, but is hard to determine. • Negative anomalies seem to correlate better than positive anomalies • The decrease in tropical cyclone activity starting at 1000AD is not reflected in the precipitation anomalies • The lack of data points in the precipitation anomalies makes it hard for correlation with the statistical model Summary Overall, there is evidence for correlation between equatorial rainfall in Africa and tropical cyclone activity in the North Atlantic. Theoretically, there should be a nice correlation between the two regions with time periods of high equatorial African rainfall correlating with areas of high tropical cyclone activity and vice versa. There are areas of local minima in the TA activity that correlate with minima’s in rainfall in Africa, specifically at 1380AD, 1150 AD, and 580 AD. Since low rainfall in Equatorial Africa would produce fewer waves that propagate out to the eastern Atlantic and provide the “seeds” for tropical cyclone development, less tropical cyclones would form. What is interesting is that this correlation does not apply to maximas. Possibly, there are other forces that are responsible for letting these equatorial waves become tropical cyclones, such as wind shear, sea surface temperatures, and multi-decadal weather/ocean patterns not investigated in this study. References Hopsch, S. B., Thorncroft, C. D., Hodges, K. and Aiyyer, A., 2007: West African storm tracks and their relationship to Atlantictropical cyclones. J. Climate, 20, 2468-2483, doi: 10.1175/JCLI4139.1 Bonnefille, R., Chalie, F., 2000: Pollen-inferredtime-series from equatorial mountains, Africa, the last 40 kyr BP. Global and Planetary Climate Change, 26, 25-50. Mann, M. E , Kozar, M. E.,. Emanuel, K. A., and Evans, J. L., 2009: Long-term variations of North Atlantic tropicalcyclone activity downscaled from a coupled model simulation of the last millennium.Journal of Geophysical Research: Atmospheres, 118, 13383-13392.